The invention is based on a fuel injection system and a method therefor as described hereinafter. In such a system, known from German Pat. No. 27 21 628, the quantity of pre-injection fuel is introduced into the combustion chambers of an internal combustion engine by way of a special fuel injection valve arrangement in the form of a double valve, separately from the amount of main injection fuel. The pre-injection fuel quantity is controlled by a separate pump arrangement and the main injection fuel quantity by a conventional fuel injection pump. The pre-injection fuel quantity is constant in the known device, but the injection time, i.e. the lead time of the injection over the main injection, can be controlled depending on the load or the rpm.
It has long been known to reduce the noise generation during the operation of a self-igniting internal combustion engine with the aid of a smaller pre-injection quantity preceding the main injection. The harsh combustion noise is created during the use of the common injection devices for self-igniting internal combustion engines because a certain quantity of fuel collects in the combustion chamber in the interval between the onset of injection and the onset of combustion. This quantity is suddenly ignited at the onset of combustion causing a steep rise of the combustion chamber pressure. This very steep rise creates the known hammering noise. On the other hand, however, the injected fuel has sufficient time during the interval between the onset of injection and the onset of combustion to mix well with the air swirling in the combustion chamber. Because of this optimal mixing the fuel consumption of the internal combustion engine is reduced.
If, prior to the injection of the main injection quantity, a small amount of fuel is injected, combustion begins "softly" with this small injection quantity. When the main injection sets in, the ignition temperature required for the introduced fuel has already been reached because of the combustion of the pre-ignition amount. The main injection quantity can then be burned in the combustion chamber as it is injected without a long ignition delay. The combustion pressure curve in such type of combustion is less steep and the noise associated with the combustion is minimal. This injection method, however, has the disadvantage that the main injection mass can no longer intensely mix with the air present in the combustion chamber before the onset of combustion. Fuel consumption and smoke generation in this combustion process are higher than in the process described above.
In the apparatus above-described having a pre-injection pump, the reduction of combustion noise is also achieved at the expense of higher consumption or even generation of smoke.